1. Field of the Invention
This invention relates to apparatus for measuring relative stability of two signals and more particularly such apparatus for providing a beat note square wave from two such mixed signals and a picket fence signal providing a time reference grid of period shorter than the beat period to an interval counter and thence to a computer containing an algorithm for calculating the exact times of the beat note upcrossings from the output of the counter.
2. Description of Related Art
In the prior art U.S. Pat. No. 4,118,665 Reinhardt resembles the subject apparatus in that phase deviations of a device under test are converted to time deviations of a sequence of pulses. In both systems these time deviations are measured by an interval counter with respect to a reference pulse train provided by a reference frequency source and a divider. There are two main differences. First, since the front end of Reinhart's system uses the quadrature method of phase detection, phase deviation between two frequency sources can be followed up only up to plus or minus 1/4 of a cycle. Since the front end of the present system uses the frequency offset method, phase deviations of arbitrary magnitude can be followed. Secondly, in Reinhardt's system the pulses to be measured necessarily remain coherent with the reference pulses, which are at the same frequency. Again, this limits the possible range of phase measurement. In the present system, because of the operation of the unfolding algorithm, the pulses to be measured need not maintain coherency with the reference picket fence pulses, which are at a higher frequency.
U.S. Pat. No. 4,027,146, Gilmore describes the design of a time interval counter that might be used as a component of the measurement system of the present invention. The present invention consists of apparatus for avoiding the dead time limitation of this type of counter, which needs to reset itself between measurements.
U.S. Pat. No. 4,000,465, Sugiyama describes a digital tachometer that holds its reading even after the instrument shaft has been removed from the rotating object. This device merely counts the number of pulses within a known gate interval whereas the present invention gives precise measurements of all the upcrossing times of a sequence of pulses.
U.S. Pat. No. 4,654,586, Evans, Jr., et al. describes a digital phase meter that measures the lag between two square waves of the same frequency by counting clock pulses between an upcrossing of one square wave and an upcrossing of the other square wave. It is a specialized time interval counter whereas the present invention shows how to use a time interval counter in a novel way.
U.S. Pat. No. 4,544,884, Hayashi describes an improvement in the art of time interval counters that operate by counting the number of clock pulses between two events. It improves the measurement accuracy by accounting for the fractional portions of the clock pulses at the beginning and end of the measurement. The present invention is concerned not with the art of interval counter design but with a novel apparatus that includes an interval counter as one of its elements.
U.S. Pat. No. 4,403,185, Charles detects when the period of a square wave falls below a predetermined threshold. Thus, its output represents only a gross two-state indication of square wave periods whereas the present invention is capable of measuring each period down to the resolution of the interval counter used.
A counter assembly by LeCroy can latch the readings of a free-running counter with the precision of one nanosecond for several input channels. The counter rolls over every 2.sup.24 nanoseconds, about 16.8 milliseconds. In effect it makes its own picket fence with that period and the same unfolding algorithm disclosed in this subject invention happens to apply. A multi-channel frequency stability measurement system built around this unit might be smaller and less expensive than one built around several interval counters in separate chassis. The LeCroy unit never has to stop and reset so that there is no dead time problem such as solved by the subect invention.